(1) Field of the Invention
The present invention relates to a liquid crystal display device capable of efficiently rejecting heat generated inside a casing.
(2) Description of the Related Art
Recently, a luminous type plasma display device and a non-luminous type liquid crystal display device have been increasingly used as the display device in place of the CRT (Cathode Ray Tube).
The liquid crystal display device employs the liquid crystal panel as the transmission type light modulation device, having a back surface equipped with a lighting device (hereinafter referred to as a backlight device) for irradiating light rays to the liquid crystal panel. The liquid crystal panel controls transmittance of the light irradiated from the backlight device to form an image.
Mainly, there are two types of the backlight device for irradiating the light to the liquid crystal panel. One is a side-light type having the light source disposed at lateral or longitudinal ends of the liquid crystal panel for irradiating the light via a light guide plate which serves to output the incident light from the side surface toward the plane, and the other is down type for irradiating the light from the back surface of the liquid crystal panel.
The liquid crystal display device has a feature capable of forming a thin external configuration. Recent trend demands the device to be further thin and to reduce the power consumption. If the thickness of the liquid crystal display device is reduced excessively, it becomes difficult to form the air passage for rejecting the heat generated inside the casing that forms the external configuration of the liquid crystal display device. As a result, the heat cannot be efficiently rejected, thus increasing the temperature at the heat sensitive spot.
Patent Documents 1 to 3, that is, JP-A Nos. 2006-235399, 2006-308738 and 2006-344472 disclose the use of LED (Light Emitting Diode) as the light source for the backlight device of the side-light type liquid crystal display device, and the structure for releasing the heat from the back surface of the substrate provided with the light source connected to the heat release member so as to discharge the heat generated in the LED.
The side-light type backlight device for irradiating the light to the liquid crystal panel is likely to have the light sources concentrated at end portions of the screen. It is therefore difficult to release the heat from the light source or perform cooling, for example, to control illuminance of the light source in accordance with the image signal, and enlarge the structure compared with the down type backlight device.
Especially when the liquid crystal display device is intended to have its size diversified from small size to large size, the light guide plate optically designed and sized in accordance with the screen size is required. It is difficult to utilize the common parts, thus interfering cost reduction.
The down type employs more light sources than the side-light type, increasing the cost and power consumption. The down type requires the distance from the light source to the liquid crystal panel (that is, the distance of the liquid crystal panel in the thickness direction) to be increased for the purpose of suppressing uneven brightness of the image to be displayed on the liquid crystal panel, which is disadvantageous for reducing the thickness of the liquid crystal display device. Furthermore, the distance from the light source to the liquid crystal panel makes it difficult to provide high contrast ratio under the light source control.